1. Field of the Invention
The present invention relates to a highly refractive and highly transparent optical glass, the use of such a glass, optical elements and processes for producing the glass or the optical elements.
2. Description of Related Art
In recent years, the market trend both in optical technologies and optoelectronic technologies (fields of application of imaging, projection, telecommunication, optical news technology, mobile drive and laser technology) is increasingly in the direction of miniaturisation. This can be seen from the end products which are becoming ever smaller and requires increasing miniaturisation of the individual components and constituents of such end products. For this reason, ever more refractive glasses, i.e. glasses having a greater refractive index or index of refraction, are required. Highly refractive glasses make it possible to shorten the component or lens size by shortening the focal length of a lens element. Furthermore, small radii of curvature of the lens elements are possible, which leads to simpler production less prone to errors.
At the same time, these highly refractive glasses should have low dispersion, i.e., high Abbe numbers. A large or high Abbe number makes correction of the chromatic error in a lens system (colour deviation) possible. Glasses having high refractive indices normally have small Abbe numbers, i.e. they display relatively high dispersions.
In addition, ever more demanding quality requirements make very high internal transmissions (German “Reintransmission”) of the material necessary and it is desirable for the glasses not only to have the required optical properties but also be sufficiently chemically resistant and have very low coefficients of expansion.
Glasses having similar optical positions, i.e. the position in the Abbe diagram or chemical compositions have already been described in the prior art, but these glasses have considerable disadvantages.
Unfavourable compositions of rare earth metal oxides, WO3, TiO2, Ta2O5 and/or Nb2O5 can lead to either the optical position not being attained or else the high transmission requirements not being fulfilled. Highly refractive glasses normally display, due to the use of polarisable ions such as Bi3+ or the use of ions having absorption bands in or near the visible region (e.g. PbO, WO3 (DE 2942038) and/or TiO2), a distinct yellowish colour which is caused by shifting of the band edge to longer wavelengths, or a deterioration in the internal transmission especially in the UV region. According to the prior art, such a deterioration in the internal transmission can be avoided by, for example, using less strongly polarisable cations and ensuring that absorbing components are not used or used only in very small proportions.
In the case of high-transmission glasses, TiO2 has hitherto been avoided as glass component because TiO2 can, in addition to the absorption band in the near UV region, lose oxygen at high temperatures. As a result of such loss of oxygen reduced, coloured ions can be formed, and TiO2 can also form a brownish iron-titanate complex with iron impurities. Glasses having a very high TiO2 content are described in CN 101289276 A.
In addition, the combination of TiO2 with Nb2O5 is very challenging in process engineering terms since Nb also gives off oxygen at high temperatures and then later competes with TiO2 for the free oxygen still dissolved in the glass. If the process is not controlled precisely, brown-coloured glasses are formed. Nb2O5 and WO3, in particular, are known for shifting the UV edge of glasses significantly into the visible region.
The glasses known from the prior art, as described, for example, in DE 10 2009 047 511, also suffer from problems in production and due to the cost of the mix caused by the use of relatively high proportions of extremely expensive raw materials Ta2O5, WO3 and GeO2. Owing to the high density of the glasses of the prior art and the small processing range in combination with a very steep viscosity curve, striae (German “Schlieren”), in particular volume striae, can be formed in production and are disadvantageous in further processing to produce optical elements.
Highly refractive optical glasses in this position range frequently belong to the lanthanum borate glass system. This is known for its steep viscosity curves, its tendency for severe surface and interface crystallisation to occur and for the attack on refractory materials, in particular SiO2 (by the B2O3 components). Dissolution of SiO2 in the glass can likewise lead to severe striae and also a decrease in the refractive index of the glass and thus to known problems, so that this glass system usually has to be melted in noble metal crucibles such as Pt or Pt/Ir crucibles to avoid this effect. On the other hand, due to introduction of particles or dissolved Pt, Pt leads to shifting of the absorption edge into the visible and to an increased crystallisation tendency (in the case of particles), in particular during repressing of the glasses.
In many cases, a high content of network formers such as B2O3 (DE 2756161A, DE 10227494A, JP 2011-246337A) and SiO2 leads to a decrease in the achievable refractive index nd. Furthermore, at particular combinations of the rare earth metal oxides and the classical network formers, either the desired combination of transmission and optical position cannot be achieved or the glass system has an even stronger tendency to crystallize, which results in an unacceptably high reduction in yield and the glasses cannot be produced economically. For these reasons, it was hitherto assumed in the prior art that the borate content always has to be greater than the SiO2 content (EP 1433757 A by Hoya) or alternatively the glass must not contain any SiO2.